This small reserve (0.5 km2) is the only part of the famous Slavonian Quercus robur (pedunculate oak) flood plain forests where Q. robur has not been logged. It was protected in 1929. There had been some cutting before that, but not involving Q. robur. Prašnik is located at an elevation of about 100 metres amid the flood plains of the River Sava. In the past it was flooded for about three months in the year. Although there has been almost no cutting, the reserve is not in its natural state: the Sava is now behind dikes and the cessation of flooding has changed the nature profoundly. Carpinus betulus (European hornbeam) is invading the forest, which prevents the regeneration of Q. robur. 40 years ago C. betulus was still very scarce, but now the forest is mainly composed of very large Q. robur here and there, and dense young C. betulus. It can be seen from the form of the approx. 300-year-old Q. robur that the forest has been more open. Annual precipitation is 910 mm and average annual temperature 11°C.

Prašnik is almost unknown to the public and only foresters and scientists normally go there. However, there are no restrictions on entry, though there may still be land mines from the Croatian War. A few safe trails have been marked in the forest.